1. Field of the Invention
The present invention generally relates to a method of measuring a quantity of oxygen contained in silicon and, in particular, to a quite novel method of measuring oxygen in silicon that provides a handy, speedy and accurately controlled analysis of highly pure single crystalline silicon which is a constituent ingredient in the manufacturing process of, for example, a semiconductor silicon wafer.
2. Description of Related Art
Methods of measuring an absolute quantity of oxygen in silicon have included an activation analysis using charged particles and a vacuum-fusion method. Methods of measuring a relative quantity have included secondary ion mass analysis and infrared absorption analysis.
Of these various kinds of methods, the former three methods require remarkably large and expensive facilities or a long measuring time and much trouble. Accordingly, they are unsuitable for control analysis on an industrial level and hardly used practically; that is, they have been used merely in analysis at an investigation level. Accordingly, in most cases, infrared absorption analysis which merely requires a relatively handy apparatus and measuring operation has been used for control analysis in industry.
However, infrared absorption analysis also has at least the following problems. Since infrared absorption analysis measures a relative quantity, a difficulty in accuracy of measurement occurs in that a relatively large error of measurement is apt to be produced by a thickness effect of the silicon sample, that is, if a thickness of the silicon sample is not strictly equal to that of the standard sample. Also, it is natural in an infrared ray-opaque silicon sample (such as a silicon wafer doped with phosphor, boron, antimony and the like) which has been recently developed for a defect to occur in that infrared absorption analysis cannot be applied at all. Accordingly, in the case where such an infrared ray-opaque silicon sample is an object to be measured, the investigating method, such as secondary ion mass analysis, might be attempted in place of infrared absorption analysis. However, that type of analysis is remarkably disadvantageous in economy, measuring efficiency (usually about 1 hour/1 analysis), operation and the like.
So, the present inventors carried out various kinds of investigation aimed at the development of a method of measuring oxygen in silicon which is a more handy, practical control analysis in place of the conventional infrared absorption analysis and sufficiently applicable even in the case where an infrared ray-opaque silicon sample is an object to be measured. The result was the invention of a method of measuring oxygen in silicon using an extraction analysis of gases in a metal by a heat melting method, which has been already practiced by Horiba, Ltd. And, the present inventors carried out the investigation of the possibility of such method.
According to such a method of measuring oxygen in silicon, as roughly shown in FIG. 2, a graphite crucible "c" is capable of being adjusted in temperature by putting it between an electrode "a" and an electrode "b" in a pressed manner and electrifying it (i.e., passing an electric current "i" through it). Heat is thereby generated at an appointed high temperature to degas the graphite crucible "c" itself. Then, a silicon sample "s" is thrown into the graphite crucible "c" together with a flux metal "m" (for example, metals such as nickel and tin for use in a metallic bath) to extract oxygen contained in the silicon sample "s" in the form of gases combined with carbon (for example, carbon monoxide gas). Subsequently, the extracted gases are introduced into a known gas-concentration analyzer system (not shown) to detect the gas concentration, whereby a quantity of oxygen contained in the silicon sample "s" is measured. That is to say, the same procedure as in the case where an object to be measured is for example iron is adopted.
The method of measuring oxygen in silicon using such a heat melting type gas extraction analysis can be applied also to an infrared ray-opaque silicon sample in principle and can very simply measure a quantity of oxygen contained in the silicon sample. However, various kinds of problems have occurred as follows:
(a) The following reaction makes progress between the graphite crucible "c" and the silicon sample "s" thrown into the graphite crucible "c" to locally produce deteriorated portions (SiC portions having a large electric resistance) in the graphite crucible "c." Therefore, a remarkably large amount of Joule's heat is generated in the deteriorated portions by the electric current "i" passing through the graphite crucible "c" to locally heat the graphite crucible "c" to an abnormally high temperature. EQU C+Si.fwdarw.SiC
Accordingly, a reaction makes progress between oxygen (so called metal oxide) contained in the graphite crucible "c" itself and a part of the SiC as follows: EQU SiC+O.fwdarw.Si+CO
As a result, disadvantages occur in that a blank gas (carbon monoxide gas) leads to a great error of measurement, and the graphite crucible "c" is damaged.
(b) Since also oxygen contained in the flux metal "m" is simultaneously extracted when the silicon sample "s" is heated, a highly pure flux metal "m" containing a very small amount of oxygen must be used. Also, this can lead to a great error in measurement.
(c) Also, an error of measurement due to the rapid evaporation of the silicon sample "s" within the graphite crucible "c" and a greater affect of the gas (carbon monoxide gas) extracted from the vaporized silicon is apt to be produced.
That is to say, practically good results cannot be readily obtained.